Link adaptation by channel bundling in a wireless communication system

ABSTRACT

The present invention relates to a method of reducing interference in a communication system supporting multiple radio channel communication scheme and wherein a communication link can be established between a transmitter and a receiver. In the method the transmitter first determines ( 301 ) interference level at the transmitter, and then based on the determined interference level, performs link adaptation, wherein the link adaptation comprises applying ( 307 ) channel bundling for adapting the communication link so as to transmit simultaneously on at least two radio channels from the transmitter to the receiver.

TECHNICAL FIELD

The present invention relates to a method of reducing interference in acommunication system. More specifically the invention relates to a newlink adaptation scheme applicable in various communication systems. Theinvention also relates to a corresponding computer program product andcommunication device.

BACKGROUND OF THE INVENTION

In wireless communication networks, the conditions of the wirelesschannel change continuously. In many cases the changes are so remarkablethat a reaction from the communication network is required in order tocontinue fulfilling the transmission requirements. One possible reactionis link adaptation. Link adaptation is a technique applied in networks,where different Physical layer (PHY) modes are available for datatransmission, and conventionally it consists of a function that choosesthe appropriate PHY mode, under the given channel conditions. Physicallayers provide multiple data transmission rates by employing differentmodulation and channel coding schemes.

In modern wireless local area network (LAN) and personal area network(PAN) standards, a variety of PHY modes, consisting of a modulation andcoding scheme, are available for the data transfer. Current linkadaptation schemes choose one of the available PHY modes for theoncoming data transfer, based on some decision variables such as thepacket error rate (PER) and/or measurements on channel quality,performed by the mobile stations (MSs) during idle times.

Publication entitled “Goodput Analysis and Link Adaptation for IEEE802.11a wireless LANs” by Daji Qiao et al., IEEE transactions, vol. 1,issue 4, October-December 2002, pages 278-292 discloses an example of alink adaptation algorithm. This and similar algorithms perform well forIEEE 802.11a/e wireless LANS (WLANs), but they are designed for a singlechannel protocol, and are suboptimal when used in multichannel systems.Especially when multiple access interference (MAI) among different,simultaneous transmissions of different users is present, the adjustmentof the PHY mode used by one of the active users on one channel, andconsequently the transmission power used, changes the interferencesituation in the network. This results in affecting other links thatneed to adjust their transmission characteristics. As a consequence, thenetwork might become instable. Thus, there is a need for an improvedlink adaptation algorithm that performs well in multichannel systems.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is proposed a methodof reducing interference as recited in claim 1.

Especially in asynchronous code division multiple access (CDMA)networks, the performance of the receiver is highly affected by thenumber of simultaneous users, the relative delay between theirtransmissions and the power of each interferer. By applying channelbundling, the transmitter occupies at least two channels, and reducesthe number of potential interferers to its receiver by one in case twochannels are used. By occupying multiple channels, the transmitterblocks these channels so that other users cannot use these channelswhile the transmitter is occupying these channels. Thus, the performanceof a multi user detector (MUD) is enhanced, leading to higherinterference suppression and higher signal to interference and noiseratio (SINR) at the interference detector. In some cases, this SINRenhancement is enough to keep the PHY mode unchanged and profit fromlower delays.

Furthermore, channel bundling offers more capacity to the link and incase a lower PHY mode is used, this method can consequently compensatefor the longer transmission time required.

According to a second aspect of the invention there is provided acomputer program product comprising instructions for implementing themethod according the first aspect of the invention when loaded and runon computer means of the transmitter.

According to a third aspect of the invention there is provided acommunication device as recited in claim 10, the device being arrangedfor implementing the method according to the first aspect of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent fromthe following description of non-limiting exemplary embodiments, withreference to the appended drawings, in which:

FIG. 1 is a block diagram of the environment, wherein the teachings ofthe invention may be applied;

FIG. 2 is a block diagram of the adaptation block capable of applyingchannel bundling in accordance with the present invention;

FIG. 3 is a simplified flow chart in accordance with an embodiment ofthe present invention;

FIG. 4 shows four transmission channels along a timeline, where twochannels are bundled for simultaneous transmission;

FIG. 5 shows four transmission channels on which data transmissionstarts at different time instants;

FIG. 6 shows four transmission channels on which data transmissionstarts simultaneously; and

FIG. 7 is diagram showing simulation results.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following description some non-limiting exemplary embodiments ofthe invention will be described in more detail in the context of codeddistributed coordination function (C-DCF) in a multi carrier CDMA(MC-CDMA) system. However, it is to be understood that the invention isnot restricted to this environment, but the teachings of the inventionare equally applicable in communication systems employing other multipleaccess schemes such as frequency division multiple access (FDMA), as faras they employ multiple channel structure.

An example of a communication system that operates in accordance withthe MC-CDMA scheme is IEEE 802.11a/e which has become a worldwide WLANstandard. From the point of view of medium access control (MAC) layer,by the use of MC-CDMA, the frequency channel is divided in severalchannels separated by different spreading sequences. The differentchannels can also be referred to as codechannels (cchs). The DCF, as thebasic access mechanism of the IEEE 802.11 MAC, achieves automatic mediumsharing between compatible devices through the use of carrier-sensemultiple access with collision avoidance (CSMA/CA). Before the devicestarts transmission, it senses the wireless medium to determine if it isidle. If the medium appears to be idle, the transmission may proceed,otherwise the device will wait until the end of in-progresstransmission. The CSMA/CA mechanism requires a minimum specified spacebetween contiguous frame transmissions. A device will ensure that themedium has been idle for the specified inter-frame interval beforeattempting to transmit.

The distributed inter-frame space (DIFS) is used by devices operatingunder the DCF to transmit data frames. A device using the DCF has tofollow two medium access rules: (1) the device will be allowed totransmit only if its carrier-sense mechanism determines that the mediumhas been idle for at least DIFS time; and (2) in order to reduce thecollision probability among multiple devices accessing the medium, thestation will select a random backoff interval after deferral, or priorto attempting to transmit another frame after a successful transmission.The device which has a data packet to transmit, draws a random numberbetween 0 and contention window, which determines the duration of thebackoff timer in number of timeslots.

FIG. 1 shows an environment, where the teachings of the presentinvention can be applied. In FIG. 1 there are shown wirelesscommunication devices 101, in this example mobile phone handsets 101. Inthis example these devices 101 are arranged to communicate in accordancewith the IEEE 802.11a standard with an access point 103. Thus, thecommunication between the wireless devices takes place over the air. Theaccess point 103 may further relay the requests sent by the devices 101to a server 105. A wired connection can be arranged between the accesspoint 103 and the server 105. In the arrangement of FIG. 1 only onewireless device is entitled to communicate at a time on a certainchannel with the access point 103. For this purpose, the wirelessdevices take advantage of the CSMA/CA as explained above. However, thewireless devices 101 may simultaneously communicate with the same accesspoint by using other channels or with other access points using the samecommunication channel and the same or different communication protocolthereby causing interference in the communication system.

FIG. 2 shows a simplified block diagram of the link adaptation block 201that is part of the communication device 101. Other elements of thecommunication device 101 are not described in this context, since theyare known for a skilled man in the art. The link adaptation block 201comprises three blocks in FIG. 2, namely a channel bundling block 203, aPHY mode adaptation block 205 and a power control block 207. As can beseen from the figure, these blocks are arranged to communicate with eachother so that when one of these blocks is making a decision, theoperation of the other blocks can be taken into account in the decisionmaking. The link adaptation block 201 takes as an input determinedinterference level and outputs the number of transmission channels, withselected PHY mode and transmission power level. According to anembodiment, the determined interference level is input to all of thesethree blocks. The operation of these blocks is later described in moredetail.

FIG. 3 shows a flow chart depicting the method of reducing interferencein a communication system in accordance with an embodiment of theinvention. The method depicted in the flow chart of FIG. 1 can beapplied in a wireless device 101, which intends to transmit data to theaccess point 103.

In step 301 the wireless device 101 determines the interference level onthe radio channel it intends to use for transmission. For determiningthe interference, it may for instance measure signal to interferenceratio (SIR) of a pilot signal received from the access point 103 or PERor any other relevant value. The determined interference level is thenfed to the link adaptation block 201. Then in step 303, the interferencelevel is compared to a pre-defined threshold value. The comparison canbe done in each of the blocks contained in the link adaptation block201. Alternatively there could be one element for the comparison so thatthis element would then inform the other blocks in the link adaptationblock 201. The threshold value could also be adjusted dynamicallydepending on some variables in the network. For instance whentransmitting data more interference is tolerated than when transmittingspeech. Thus, the interference threshold may depend on the type oftransmission. If the interference level is below the threshold, thenthere is no need for link adaptation and the transmission can take placein step 305 once there are free resources using the intended modulationand coding scheme.

However, if in step 303 it is determined that the interference level isnot below the threshold value, then channel bundling is applied in step307 by the channel bundling block 203. The principle of channel bundlingis shown in FIG. 4. In this case the data is transmitted simultaneouslyon channels 2 and 3 as shown in the figure. Channel 1 is not suitablefor transmission, since during the backoff period it was detected thatthere was another terminal already transmitting on this channel. On theother hand channels 2 and 3 are free because during the backoff period,which in this example is four timeslot periods, these channels weredetermined to be free. In this example channel 4 is not suitable fortransmission during the desired period as it was occupied. Even if inthe example of FIG. 4, the transmission on channel 4 ends roughly at thesame time as the transmissions on channels 2 and 3 start, this does nothave to be the case. The only thing that matters in this case is thatthe channel 4 was occupied during the DIFS and backoff periods. As theinvention is explained in the context of MC-CDMA system, in FIG. 4different coded channels are identified by their respective spreadingcodes. Of course the wireless device 101 could transmit on more than twochannels simultaneously if need be with the condition that there areenough free channels available.

Once the decision is made to apply channel bundling, then multiplebackoff processes are started in parallel. The backoff processes do notnecessarily have to have the same backoff parameters. Usually, even ifthe backoff parameters are the same on all channels, depending on thetraffic on the channels, some backoff count-downs will end earlier thanothers. The device 101 may then start multiple transmissions in parallelon the different channels, on which the back-off has been completed.There are two alternatives:

-   -   The device 101 starts transmission on each channel        independently, once a backoff has been completed. In this case        the transmissions on different channels do not usually start at        the same time. This is shown in FIG. 5.    -   The device 101 starts transmission on multiple channels in        parallel, but waits until the backoff on a certain number of (up        to all) channels has been completed to transmit the data in        parallel, e.g. at a higher “bundle data rate”. This is shown in        FIG. 6.

This procedure can be expanded for stations using n out of m (n≦m)channels, where the countdown is not interrupted in p (p≦n) cases,leading the station to start parallel transmissions on d (d≦p) channels.It is to be noted that in FIGS. 5 and 6, the backoff periods aredifferent for different channels, whereas in FIG. 4 a common backoffperiod is chosen for several channels.

Then in step 309 it is determined whether there is need to change thePHY mode, i.e. the modulation and/or coding scheme. If there is no needto change the PHY mode, then the data can be transmitted on the selectedchannels. On the other hand, if in step 309 it is determined that PHYneeds to be changed, then in step 311, the wireless device 101 thatintends to transmit the data changes the PHY as decided in step 309.This is done by the PHY mode adaptation block 205. Thus, if both thechannel bundling adjustment and the change of PHY mode are done, themethod can be referred to as a two dimensional link adaptation method.Then the data is transmitted in step 305. Also, if in step 309 it wasdetermined that there is no need to change the PHY, then data istransmitted in step 305 without changing the PHY mode. After this theprocedure comes to an end or it may restart again by determining theinterference level in step 301.

The IEEE 802.11a has eight PHY modes as shown in Table 1. For instancefor a link that operates by using a PHY mode 3, i.e. QPSK modulationwith code rate ½ can be switched to operate on two parallel channels inmode 1 with BPSK modulation and code rate 1/2. In both cases the finaldata rate is 12 Mbps. Thus, by using the method of the present inventionthe data rate can be kept constant, if this is wanted. It is of coursealso possible to transmit simultaneously on multiple channels withincreased PHY mode. This would mean that the transmission would becompleted in a much shorter time.

TABLE 1 Eight PHY modes of the IEEE 802.11a. Mode Modulation Code rateData rate 1 BPSK ½  6 Mbps 2 BPSK ¾  9 Mbps 3 QPSK ½ 12 Mbps 4 QPSK ¾ 18Mbps 5 16-QAM ½ 24 Mbps 6 16-QAM ¾ 36 Mbps 7 64-QAM ⅔ 48 Mbps 8 64-QAM ¾54 Mbps

FIG. 7 shows some simulation results. SINR at the detector, i.e. at thedevice 101, is shown as a function of the power of the first interfererfor five different values of the second interferer's power. The SINRvalue in the diagram is the average over 10 000 runs with differentrelative delays among concurrent user transmissions. The graphs showthat performance decreases almost linearly as a function of theinterfering power. One important observation is that the MUD manages toprovide a positive SINR even in the case when all the three interferingsignals are 5 dB higher than the carrier strength. This demonstrates theinterference suppression ability of the detector in the presence of highmultiple access interference (MAI).

The interference reduction by channel bundling affects positively allother links in the network, having as a minimum consequence thereduction of the applied transmission powers. The outcome is overallinterference reduction and power saving at many devices 101.

In cases when a connection cannot be driven with the chosen PHY mode, itis beneficial according to the two dimensional link adaptation scheme touse channel bundling by the transmitting device before shifting to amore robust PHY mode.

The channel bundling can also be applied by a link adaptation algorithmas an alternative to power adjustment, for instance by a power controlalgorithm. Thus, when receiving from the network a power control commandto lower the transmission power, the device 101 would perform channelbundling possible simultaneously lowering the PHY mode, but withoutadjusting the transmission power. In this case the power control block207 would control the channel bundling and PHY mode adaptation blocks.Alternatively, the channel bundling can be applied by a link adaptationalgorithm in conjunction with power adjustment, for example by a powercontrol algorithm.

The invention equally relates to a computer program product that is ableto implement any of the method steps of the embodiments of the inventionwhen loaded and run on computer means of the transmitting device 101.

The invention equally relates to an integrated circuit that is arrangedto perform any of the method steps in accordance with the embodiments ofthe invention.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not restricted to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure and the appendedclaims. In the claims, the word “comprising” does not exclude otherelements or steps, and the indefinite article “a” or “an” does notexclude a plurality. A single processor or other unit may fulfil thefunctions of several items recited in the claims. The mere fact thatdifferent features are recited in mutually different dependent claimsdoes not indicate that a combination of these features cannot beadvantageously used.

A computer program may be stored/distributed on a suitable mediumsupplied together with or as a part of other hardware, but may also bedistributed in other forms, such as via the Internet or other wired orwireless telecommunication systems. Any reference signs in the claimsshould not be construed as limiting the scope of the invention.

1. A method of reducing interference in a communication systemsupporting multiple radio channel communication scheme, wherein acommunication link is established between a transmitter (101) and areceiver (103), the method comprising: determining (301) an interferencelevel at the transmitter; and based on the determined interferencelevel, performing a link adaptation, wherein the link adaptationcomprises applying (307) a channel bundling for adapting thecommunication link so as to transmit simultaneously on at least tworadio channels from the transmitter (101) to the receiver (103).
 2. Themethod according to claim 1, wherein the link adaptation furthercomprises adjusting (311) a physical layer mode, wherein the physicallayer mode is defined by at least one of a modulation scheme and acoding scheme.
 3. The method according to claim 2, wherein the channelbundling is performed before the physical layer mode is adjusted.
 4. Themethod according to claim 1, wherein the transmitter (101) receives apower control command from the communication system, the method furthercomprising receiving the power control command from the communicationsystem and, based on the power control command, applying the channelbundling without adjusting a transmission power of the transmitter(101).
 5. The method according to claim 1, wherein the transmitter (101)receives a power control command from the communication system, themethod further comprising receiving the power control command for thecommunication system and, based on the power control command, applyingthe channel bundling in conjunction with a power adjustment of thetransmitter (101).
 6. The method according to claim 1, wherein after thechannel bundling has been applied, the transmission startssimultaneously on multiple radio channels.
 7. The method according toclaim 1, wherein after the channel bundling has been applied, thetransmission starts at different time instants on different radiochannels.
 8. A computer program product comprising instructions forimplementing the steps of a method according to claim 1 when loaded andrun on computer means of the transmitter (101).
 9. A communicationdevice (101) in a communication system supporting multiple radio channelcommunication scheme, the communication device comprising: unit fordetermining interference level at the communication device (101); andlink adaptation unit (201) arranged, based on the determinedinterference level, to perform a link adaptation for reducinginterference in the communication system, wherein the link adaptationcomprises applying a channel bundling for transmitting simultaneously onat least two different radio channels.
 10. A computer-readable storagemedium having executable instructions for reducing interference in acommunication system supporting multiple radio channel communicationscheme, wherein a communication link is established between atransmitter and a receiver, when executed, the acts comprising:determining an interference level at the transmitter; and based on thedetermined interference level, performing a link adaptation, wherein thelink adaptation comprises applying a channel bundling for adapting thecommunication link so as to transmit simultaneously on at least tworadio channels from the transmitter to the receiver.